There are large numbers of computing devices that have touch-sensitive screens which allow users to interact using touch gestures directly on the device's screen. Examples include smartphones, tablet computers, large interactive surface computers and touch-sensitive displays for desktop computers. Although for some applications, this interface provides an adequate user input mechanism, for some applications and for some users (e.g. users with a visual impairment), the flat, continuous interaction surface (the device screen) can be difficult to use as there are no physical boundaries or tactile feedback to help guide and keep the user's fingers in the correct place. In addition, since the device's screen is used both as an input device and a display device (i.e. as an output device), the user's fingers may obscure areas of the screen whilst interacting with an application.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known user input devices.